1. Field of the Invention
The present invention relates to a binarization-use-pattern generating method and printing apparatus and, in particular, to a binarization-use-pattern (dither pattern table) generating method and printing apparatus for use in a binarizing process of converting a multi-value image (continuous tone image), such as a natural image, a graphic image, and a color character, to a binary image.
2. Description of the Related Art
To output from an output device, such as a printer, a multi-value image, such as a natural image, a graphic image, and a color character, obtained as being captured or generated from an input device, such as a scanner, a binarizing process is required so as to represent this multi-value image with ON/OFF of a color material (such as ink or toner; hereinafter, also referred to as dots).
As one binarizing technique, dithering is known. Dithering is a technique of comparing the magnitudes between a value of a specific pixel (target pixel) in a multi-value image and a preset threshold to determine a gradation density of picture elements and determining ON/OFF of dots for the pixels according to the gradation density of the picture elements by using a binarization-use pattern provided in advance. Here, dither (halftone dots) refers to a look-up table for use in converting picture elements with multi-levels of gradation to a “binary image (indicating whether to apply ink or not)”.
In dithering, clustered dithering has been known in which, as the gradation density of picture elements increases, a color material is applied to (lit at) pixels in order of increasing distance from a specific point as a center, thereby representing the gradation of the picture elements. In clustered dithering, a dot gain (ink spreading) is small, and the color material is stably fixed on a printing medium. Therefore, when the amount of color material to be applied to each pixel is subtle, this clustered dithering is preferable for use than diffused dithering, and techniques of generating a binarization-use pattern for clustered dithering have been developed.
For example, Japanese Patent No. 4000255 (hereinafter, referred to as “Patent Document 1”) by the inventors of the present application discloses a binarization-use-pattern generation technique in which, to generate a clustered binarization pattern with a desired angle and number of pixels, basic pattern shapes are generated as tetragons formed of four points of a point A(c, 1), a point B(a+c, b+1), a point C(0, d+1), and a point D(a, b+d+1) specified with arbitrary parameters a, b, c, and d (where a, b, c, and d are integers) satisfying a condition of the number of pixels n=ad+bc specified; from among the generated tetragons, a tetragon having an angle closest to a specified angle is selected as a basic pattern; a lighting order of pixels forming the basic pattern is determined; and a rectangular pattern that functions as a binarization-use pattern is generated based on the basic pattern.
However, when the look-up table to be used for converting a binary image is generated in an unreliable manner, problems occur such that binarization cannot be made or a pattern not present in the original image (moire) appears on a print image.
Here, in the technology disclosed in Patent Document 1, clustered dither (halftone dots) in general printing is generated based on calculation for output with high-definition print-image quality. Also for moire occurring as a result of printing, halftone dots can be advantageously generated through calculation for quick handling. However, there are yet problems as follows.
For example, even when a color chip for fixing 10% ink and a color chip for fixing 90% ink are superposed upon each other, it is disadvantageously impossible to achieve a color chip as closest to solid printing (100% ink) as possible. Since designing required for printing is performed by humans, the property is necessarily required for printing process. Unless this problem is solved, it causes reprinting because “the color as intended is not reproduced”.
This problem is serious particularly in the case of 50% ink, where it is important that an area where ink is applied is equal to a paper-only area where no ink is applied. However, since “ink spreading (dot gain)” is present on print in actual use, if the ink fixing area is tried to be achieved as accurately as numerals indicate, it is required to provide a one-dimensional look-up table for converting numerical values for each level of gradation and to make a correction so that the printing result has the ink fixing area specified by electronic data.
However, to make this possible, it is presumed that the ink fixing region is varied according to the value of input electronic data. If ink is not fixed or the ink fixing region is not varied, a gradation crush disadvantageously cannot be prevented with whatever measures taken. FIG. 1 is a drawing of a basic pattern of halftone dots generated based on Patent Document 1 as a conventional technology (with conditions that the number of picture elements (the number of pixels) is 72 and the screen angle is 45 degrees).
As depicted in FIG. 1, according to the conventional technology (Patent Document 1), a high-definition clustered dither pattern for use in printing a monochrome or color image can be automatically generated by a computer. However, there are various problems. For example, even with the technology of Patent Document 1, the ink fixing area cannot be completely controlled with the binarization-use pattern based on this basic pattern. As adjacent ink fixing regions are closer to each other, the ink fixing regions are adhered to each other due to surface tension, and therefore an abrupt increase in the ink fixing region and shape due to physical properties cannot be controlled any longer even if the value of the electronic data is changed. In particular, since a numeral of 50% represents a numerical value often used in designing by designers, if color reproduction cannot be controlled at 50%, an impression is made that whole printing is not good, leading to “a request for reprinting from a designer” in printing process and disadvantageously decreasing printing profitability.